text: While an isotope effect is the physical tendency for stable isotopes to distribute in a particular way, the isotopic fractionation is the measurable result of this tendency. The isotopic fractionation of a natural process can be calculated from measured isotope abundances. The calculated value is called a "fractionation factor," and allows the effect of different processes on isotope distributions to be mathematically compared. For example, imagine a chemical reaction Reactant → Product. Reactant and Product are materials that both contain the element X, and X has two stable isotopes, X (the heavy isotope, with a mass of A) and X (the light isotope, with a mass of B). The fractionation factor for the element X in the reaction Reactant → Product is represented by the notation 
  :α. α is calculated as follows:
  :α = (δX + 1)/(δX + 1)
  Fractionation factors can also be reported using the notation ε, which is sometimes called the "enrichment factor" and is calculated as follows:
  :ε = α − 1
  Like δ values, ε values can be reported in per mille by multiplying by 1000.